Many implantable medical devices include components that are deployed in particular areas within a human or animal body. In one example, an electrical sensor deployed proximate to a muscle senses activation of the muscle or within an organ or vessel senses physiological parameters. In another example, a neurostimulator deployed proximate to targeted tissue includes electrodes that deliver electrical stimulation therapy to the tissue.
The continuous monitoring of the physical status of patients having chronic illness is essential for ensuring optimal therapy and may be performed by such implantable medical devices. For example, patients having heart diseases benefit from monitoring of their cardiac and circulatory status to adjust their therapy, which may include medication drug therapy or pacing stimulation therapy, to predict acute worsening, and to prevent decompensation. While various methods may be used for monitoring cardiovascular status, implantable measuring devices are preferred over other methods in many cases. Such implantable measuring devices can determine the relevant variables directly in the body and therefore may operate more precisely and reliably than external methods. For example, the blood pressure may be directly measured by such devices within the cardiovascular system and represents an important variable for monitoring cardiovascular status.
With these and other implantable devices, it can be desirable that one or more components remain substantially anchored, so that the components will be less likely to migrate from the desired site of sensing or therapy. Devices that restrict migration of an implanted medical device or a component thereof are called “fixation structures.” Fixation structures can anchor a medical device to an anatomical feature, such as an organ or a bone. Fixation structures do not necessarily restrict all motion of the implanted device or its component, but generally reduce the motion of the device or component so that it remains proximate to a desired location. There have been many approaches that address fixation of medical devices. Some devices employ fixed protrusions such as tines to engage body tissue. Other devices use helical securing structures and still others use barbs to hold an implanted medical device in place. Some mechanical fixation structures employ adhesive properties to hold devices in place. When medical devices such as sensors are implanted within vessel walls, they can provide useful information regarding cardiovascular status. However, existing implantable medical devices may become encapsulated as the vessel reacts to the device and tissue grows around the device. Such encapsulation can interfere with the functioning of the device.